Concrete shield design

Attenuation of Radiations. 1. Primary y-Ray Attenuation. In checking a shield it is necessary to treat separately the y rays and the neutrons which are incident on the inner face of the concrete. The disposition of the y-ray problem was accomplished in the following manner  

The heat production in the thermal shield due to (1) the absorption of capture y rays foriied in the iron and in the graphite, and (2) the absorption of y rays from the core hcs been reported in detail. These reports give a valnei of 6.7 x 10 watts/cC produced at the outer face of the iron per unit thermal neutron flux at the inner face. It is estimated that less than half of this is due to y rays from the core. 

Since = 0.235 cm for 8-Mev y rays in iron (the minimum value according tp (]BNL-421), the y-ray flux at the outer face of the thermal shield is 

This states that at any point on the outside of the thermal shield the y-ray energy flux is just 0.018 times- the neutron flux at the corresponding point on-the inside face- of the thermal shield. - An examination of the calculations shows that the variation of the neutron flux in the immediate vicinity of the point.of interest has a negligible effect on the value of the y-ray flux. 

A face of the iron thermal shield may be considered to be a slab surface with a uniform volume source of y rays of strength y rays/cc-sec. The resultant y-ray energy flux in the concrete is described by the expression  

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CP-1 was assembled in an approximately spherical shape with the purest graphite in the center. About 6 tons of luanium metal fuel was used, in addition to approximately 40.5 tons of uranium oxide fuel. The lowest point of the reactor rested on the floor and the periphery was supported on a wooden structure. The whole pile was surrounded by a tent of mbberized balloon fabric so that neutron absorbing air could be evacuated. About 75 layers of 10.48-cm (4.125-in.) graphite bricks would have been required to complete the 790-cm diameter sphere. However, criticality was achieved at layer 56 without the need to evacuate the air, and assembly was discontinued at layer 57. The core then had an ellipsoidal cross section, with a polar radius of 209 cm and an equatorial radius of309 cm [20]. CP-1 was operated at low power (0.5 W) for several days. Fortuitously, it was found that the nuclear chain reaction could be controlled with cadmium strips which were inserted into the reactor to absorb neutrons and hence reduce the value of k to considerably less than 1. The pile was then disassembled and rebuilt at what is now the site of Argonne National Laboratory, U.S.A, with a concrete biological shield. Designated CP-2, the pile eventually reached a power level of 100 kW [22]. 

The concrete block walls of the cell housing the generator tube and associated components are 1.7 meters thick. The facility also includes a Kaman Nuclear dual-axis rotator assembly for simultaneous transfer and irradiation of reference and unknown sample, and a dual Na iodide (Nal) scintillation detector system designed for simultaneous counting of activated samples. Automatic transfer of samples between load station to the rotator assembly in front of the target, and back to the count station, is accomplished pneumatically by means of two 1.2cm (i.d.) polyethylene tubes which loop down at both ends of the system and pass underneath the concrete shielding thru a pipe duct. Total one-way traverse distance for the samples is approx 9 meters. In performing quantitative analysis for a particular element by neutron activation, the usual approach is to compare the count rates of an unknown sample with that of a reference standard of known compn irradiated under identical conditions... 

It is desirable to install specially designed reprocessing equipment behind massive concrete shielding (sometimes as much as 1.5 m thick) to protect personnel from... 

The design of SMP has been developed largely around the concept of remote operation with appropriate local or bulk shielding provided around the process stages with relatively high dose rates. For example, the fuel assembly line operations are remotely operated and are enclosed in a concrete shielded room of appropriate thickness. 

In the proposed design the processing plant is a concrete-lined pit which has a capacity of about 100,000 gallons. It is located outdoors and the top of the pit is formed of rolling concrete shielding blocks. The plant is operated dry but when maintenance is necessary the cells can be flooded with water. We have developed tools for the simple maintenance jobs which we know will be necessary and, in general, are very enthusiastic about the simplicity of design which is possible because of the maintenance philosophy. 

Protection of HCF personnel from potentially lethal radiation exposures Zone 2A canyon physical structures (concrete walls, shield steel, shielding windows) Worker safety Provide radiation protection such that worker exposures in continuously occupied areas under normal and abnormal conditions are in accordance with 10 CFR 835 Shield design (Design Feature) Radioactive material control (Administrative Control)... 

The reactor core and the SG are housed in two steel pressure vessels that are connected by a connecting vessel. Inside of the connecting vessel, the hot gas duct is designed. All of the pressure retaining components, which comprise the primary pressure boundary, are in touch with the cold helium of the reactor inlet temperature. The primary pressure boundary consists of the reactor pressure vessel (RPV), the SG pressure vessel (SGPV), and the hot gas duct pressure vessel (HDPV), which all are housed in a concrete shielding cavity as shown in Fig. 14.8. 

The Group 6 Shield illustrates a unique spherical design which is small and portable for use with laboratory quantities of primary explosives. The Group 5 Shield demonstrates the modular design concept that makes suppressive shields an attractive alternative to inflexible concrete barricades. 

There have been two major accidents (Three Mile Island in the United States and Chernobyl in the former Soviet Union) in which control was lost in nuclear power plants, with subsequent rapid increases in fission rates that resulted in steam explosions and releases of radioactivity. The protective shield of reinforced concrete, which surrounded the Three Mile Island Reactor, prevented release of any radioactivity into the environment. In the Russian accident there had been no containment shield, and, when the steam explosion occurred, fission products plus uranium were released to the environment—in the immediate vicinity and then carried over the Northern Hemisphere, in particular over large areas of Eastern Europe. Much was learned from these accidents and the new generations of reactors are being built to be passive safe. In such passive reactors, when the power level increases toward an unsafe level, the reactor turns off automatically to prevent the high-energy release that would cause the explosive release of radioactivity. Such a design is assumed to remove a major factor of safety concern in reactor operation, see also Bohr, Niels Fermi, Enrico AIan-HATTAN Project Plutonium Radioactivity Uranium. 

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